UC San Diego

Bleaching susceptibility of coral varies among species, environmental conditions, and spatial patterns on a reef. As bleaching events become more frequent and severe, understanding the factors that increase resiliency against thermal stress is critical. Here, we used large scale imagery collected at the remote atoll of Palmyra in the central Pacific during the 2015 thermal stress event to investigate the role of topographically produced shade in mitigating bleaching severity. Change in planar area over two years, bleaching severity, and estimated amount of shade provided by neighboring reef structures was measured for the common coral genus Pocillopora. Colonies that bleached more severely had higher rates of mortality than colonies with less severe bleaching. Further, those colonies with less severe bleaching had a higher proportion of colonies experiencing growth. Bleaching severity was correlated with the estimated irradiance, as more daily sunlight resulted in more severe bleaching than corals experiencing less daily sunlight. Structural complexity and variability in reef topography provide regions of shade where the compound impact of temperature and irradiance is alleviated, resulting in less-severe bleaching outcomes for an abundant and cosmopolitan coral genus. While some studies have considered artificially shading reefs during warm water events, we highlight the importance of existing structural complexity in providing shade, ultimately leading to increased reef resilience in the face of a changing climate.